Vacuum chambers are widely used for many process related applications, such as optical coating, web coating, thin film coating, evaporation, magnetic material application, and load lock construction, to name only a few of the large variety of semiconductor processing techniques where a very low vacuum pressure is required. During such processing, high speed vacuum pumping may be required to evacuate a processing chamber and high speed vacuum pumping may especially be necessary to evacuate water vapor within the chamber because the partial pressure of water generally dominates the total vacuum pressure within the chamber.
Cryopumps are widely used to achieve a high vacuum (low vacuum pressure) within processing chambers. A cryopump is a pump which utilizes a cold array or other cryogenic element inside of a pump housing. The cryopump is coupled to processing chamber and the environment within the chamber is exposed to the cryogenic element. Gas particles within the chamber environment lose their kinetic energy and stick to the cryogenic element which facilitates the pumping action of the cryopump. One significant advantage of a cryopump is its high pumping speed for water vapor. Since the partial pressure of water vapor generally dominates the total vacuum pressure within the processing chamber, high pumping speed for water vapor is very important for high vacuum systems, thus making cryopumps desirable in high vacuum applications.
However, there are several drawbacks to existing cryopumps. Although cryopumps achieve a high pumping speed for water vapor, the full speed of the pump is often not achieved due to physical restrictions within the pump housing. FIG. 1 shows a conventional cryopump arrangement 10 attached to a processing chamber 12. The cryopump includes a tube 14, a connecting flange 16, and a valve 18 which opens and closes to expose the cryogenic element 20 of the pump to the environment of processing chamber 12. As is illustrated in FIG. 1, the dimensions of the cryopump 10 are restricted with respect to the dimensions of the processing chamber 12, As a result, the tube 14, flange 16, and valve 18 have a low conductance which reduces the effective pumping speed for water of the cryopump to generally less than half of the cryopump's original speed. This increases the time necessary to achieve a high vacuum within the processing chamber, and therefore, increases the time necessary to perform the necessary processing steps within the chamber. The increased time, in turn, increases the cost of semiconductor processing or any other processing.
Attempts have been made to overcome the low conductance of conventional cryopump construction. One attempted solution has been to place a stationary cryogenic element, such as a cryogenic panel, within the processing chamber. Referring now to FIG. 2, a cryogenic panel 22 might be placed inside of the processing chamber 12 to eliminate the low conductance of the tube, flange, and valve of FIG. 1. In the pumping arrangement of FIG. 2, the pumping speed of the cryopump is proportional to the area of the cryogenic panel 22 and is not restricted by the low conductance of the housing or the valve assembly surrounding the panel. With the configuration of FIG. 2, a very high pumping speed is achieved.
However, while positioning the cryogenic panel within the processing chamber increases the pumping speed, it creates another drawback. The stationary cryogenic panel 22 is always exposed to the processing chamber environment. When the processing chamber 12 is vented or open to atmospheric pressure, such as to remove or insert substrates or work within the chamber or to maintain or clean the chamber, a large amount of water vapor and moisture along with other gases are introduced into the processing chamber. The moisture has a tendency to stick to a large amount of the cryogenic panel and creates ice on the panel. The temperature rise due to the ice formation disables the pumping function. Accordingly, it becomes necessary to regenerate the cryogenic panel and keep the panel above room temperature before the processing chamber is exposed to atmospheric pressure.
There is a need for a high speed vacuum pump which is not restricted by a low conductance coupling during pumping of a processing chamber in order to rapidly remove gas particles, such as water vapor, from the processing chamber to reduce the wafer processing time and the cost of the processing operation. Furthermore, there is a need for a high speed pumping apparatus which does not require continual regeneration of the cryogenic element when the processing chamber is exposed to atmospheric pressure.